Fire Department Connection Supply Pressure

The placard on the connection states the pressure the SYSTEM needs there, and the pump operator has to add everything between the pump panel and that connection: the friction loss in the supply lines and the elevation change at 0.434 psi per foot. On a short lay both are small; on a long lay to a rear-yard connection at high flow they are not, and the difference shows up as a standpipe outlet that does not make its rated pressure four floors up. The flow split across lines is where the arithmetic bites, because friction loss goes as the SQUARE of flow: two lines each carrying half the total lose a QUARTER of what one line carrying all of it would. Supplying a connection with one line when two are available is the single most common way a pump operator ends up short. A system needing 100 psi at the connection at 500 gpm through two 200 ft lines of 3 in hose loses 8.5 psi and the engine makes 108.5; put the same 500 gpm through one line and the loss quadruples to 33.9 psi and the engine has to make 133.9 -- twenty-five psi of pump capacity spent on a decision made in twenty seconds at the hydrant. A pump-panel estimate; the department's own friction-loss coefficients, the system placard, and the pump operator govern.

flow_per_line_gpm = total_flow_gpm / lines; friction_loss_psi = C x (flow_per_line/100)^2 x (length/100); elevation_psi = 0.434 x elevation_ft; engine_pressure_psi = fdc_pressure + friction_loss + elevation.

Fire department connection supply pressure from the standard fireground friction-loss form FL = C (Q/100)^2 (L/100) with the department's own coefficient for the hose in use, plus 0.434 psi per foot of elevation, by name. The pressure required at the connection is the system placard's. The department's coefficients, the placard, and the pump operator govern.

The friction-loss form and the elevation constant are public fireground hydraulics; the coefficient, flow, and lengths are the department's and the incident's own values.

Estimate. AHJ and licensed professional govern.

Field names used by the API: fdc_pressure_psi, total_flow_gpm, lines, line_length_ft, friction_coefficient, elevation_ft, flow_per_line_gpm, friction_loss_psi, engine_pressure_psi, single_line_engine_psi

• Friction-loss form FL = C (Q/100)^2 (L/100); the coefficient C is the department's own for the hose in usefireground hydraulics
• Square of flow two lines at half the flow each lose a QUARTER of what one line wouldfireground hydraulics
• Elevation 0.434 psi per foot the connection sits above the pumphydrostatics